Energy transfer in the radiolysis of cyclopentane–cyclohexane mixtures has been studied by observing the yields of HD and D2 obtained when small amounts of cyclohexane-d12 are added. Energy migration occurs from cyclopentane to cyclohexane but this transfer can be prevented by the addition of carbon tetrachloride or iodine. The relative yields of the dimers (C6H11)2, C6H9C6H11, and (C5H3)2 show that the •C6H11 and •C5H9 radicals, which are the precursors of the dimers, can abstract hydrogen atoms from the solvent and may thus change their identity. Added iodine or carbon tetrachloride removes the diffusing radicals, and the residual dimer yield, which is unaffected by solute, has a composition determined solely by the composition of the mixtures. The yields of cyclohexene and cyclopentene show evidence of both energy transfer and radical change of identity.